Fertilizer may be manufactured in a number of manners. The three primary nutrients of interest in fertilizers are nitrogen, phosphorous (usually expressed as P.sub.2 O.sub.5), and potassium (expressed as K.sub.2 O). The major source for the phosphorous component is phosphate rock. As used herein, "phosphate rock" shall refer to mineral deposits which may be processed to render the phosphate content useful for the manufacture of fertilizer. Phosphate rock is commonly derived from mineable deposits of apatite which has the general formula Ca.sub.5 (F,Cl,OH,1/2CO.sub.3)(PO.sub.4).sub.3 or as phosphorite which is a sedimentary rock containing phosphate bearing minerals. Phosphate rock is treated by acidulation to produce phosphate in a form more available to plants. Acidulation of phosphate rock is accomplished by the treatment of phosphate rock with an acid, such as sulfuric or nitric acid. The general equation for the process when sulfuric acid is utilized is: EQU 2Ca.sub.5 F(PO.sub.4).sub.3 +10H.sub.2 SO.sub.4 +(10x)H.sub.2 O.fwdarw.6H.sub.3 PO.sub.4 +10CaSO.sub.4.xH.sub.2 O+2HF
Phosphate rock deposits vary in terms of quality and quantity. The value of a deposit depends upon consideration of its BPL (bone phosphate of lime) content, which is the phosphate content of the rock expressed as percent tricalcium phosphate, Ca.sub.3 (PO.sub.4).sub.2, and the type and amount of residual impurities. In the past, the Bone Valley, Florida deposit has been the significant domestic commercial source. The original phosphate rock mined there was of such quality that it could be processed directly upon grinding and acidulation to produce phosphoric acid. The acidulation process produces a phosphoric acid-calcium sulfate slurry containing other salts which requires solid-liquid separation by filtration. As used herein and as known in the art, the term "wet process phosphoric acid" will refer to the liquid product above described which results from the acidulation process, wherein phosphate rock is reacted with sulfuric acid to produce gypsum (CaSO.sub.4.2H.sub.2 O) and an impure solution of phosphoric acid.
As the higher grade ores were depleted, it became necessary to utilize phosphate ores containing moderate to high levels of organic matter. The level of organic matter expressed as weight percent organic carbon is considered moderate when the organic carbon content of the ore is in the range of about 0.3% to about 1.5% and is considered as high above about 1.5%. Ores containing moderate to high levels of organic impurities are difficult to process or unprocessable in conventional wet process phosphoric acid plants. The difficulty or inability to process these ores is a result of the impurities present. The organic matter in combination with evolved carbon dioxide frequently gives rise to voluminous and stable froths and prevents adequate reaction control during the conversion of the ore into phosphoric acid and the by-product calcium sulfate crystals. These froths may overflow the reactor vessel resulting in the loss of significant quantities of product, as well as requiring costly clean-up. The presence or organic matter impurities may also affect the size and shape of crystals of calcium sulfate which are formed during the reaction of phosphate rock and sulfuric acid to produce wet process phosphoric acid-calcium sulfate slurries. Separation of this by-product from the wet process acid is hindered by the crystals formed, as well as the tendency of the insoluble organic residues to coat the crystals.
In an effort to render ores containing moderate to high levels of organic impurities suitable for acidulation, the industry turned to the calcining of the phosphate rock. Calcining of the rock removed major amounts of the organic impurities. Calcining has been traditionally performed in a single-stage fluidized bed reactor or rotary kiln.
In order to minimize the difficulty in controlling the single-stage fluidized bed reaction approach, MacAskill disclosed a two-stage calcining method in U.S. Pat. No. 3,995,987 issued Dec. 7, 1976, which utilized a high capacity first stage in which the temperature was crudely controlled and in which incomplete combustion occurred. The second stage was a closely controlled stage to achieve complete combustion so as to achieve thermal efficiency. This two-stage process permitted better control of temperature and more economical use of fuel. It did not address serious problems such as loss of reactivity and sulfide generation. During the thermal treatment of phosphate rock containing organic impurities, at least two side reactions may take place which, if not controlled, negate the benefits obtained by the overall reduction in the organic impurities of the ore. These deficiencies are (1) a loss of reactivity, generally a reduction in the rates of calcium or phosphorous solubility and the loss of phosphorous in the waste by-product calcium sulfate, and (2) generation of reduced sulfur species, such as sulfides, by the reaction of the organic impurities with naturally occurring oxidized sulfur compounds in the rock. These naturally occurring sulfur compounds include sulfates or organo-sulfur compounds which occur in most sedimentary phosphate ores, such as those found in the Pungo River deposit located in North Carolina and along the East Coast of the United States.
The reduced solubility of calcium and phosphorous and phosphorous loss to the by-product calcium sulfate decreases the efficiency of the acidulation process. Generation of reduced sulfur species, such as sulfides, produces compounds which are extremely corrosive in the acidulation processes. These corrosive sulfur species attack and destroy wetted metal parts found in wet process phosphoric acid manufacturing equipment. Thus, ores containing higher levels of sulfide impurities have heretofore been unusable or difficult to use in an economical manufacture of phosphoric acid because of increased cost for replacement of conventional equipment or fabrication of process equipment from expensive corrosion resistant materials. Additionally, the loss of reactivity has also adversely affected the economic processing of such phosphate rocks.
The present invention for calcining rock minimizes generation of reduced sulfur species while maintaining acceptable reactivity of the calcium and phosphorous compounds in the rock and permits significant reduction in the level of organic carbon impurities. The feature of the present invention to achieve these objects is the utilization of a multi-stage calcining process in which the conditions of the first stage are sufficient for the destruction of a major portion of the organic impurities but at which the generation of reduced sulfur species is minimized. The product of the first stage is then fed to a second stage wherein again the oxygen level, temperature and residence time are controlled to produce a product with the desired reactivity, residual carbon content and sulfide value. The temperature and residence time utilized are that combination which maximizes the surface area of the calcined product while achieving the desired levels of residual carbon and sulfides. This maximized surface area increases the reactivity of the calcined ore during acidulation. Excess oxygen is provided in the first stage to assure maximum destruction of the organic matter. Excess oxygen is also utilized in the second stage to prevent a reducing environment which would generate additional sulfides.
In the preferred embodiment, the first stage is operated at a temperature in the range of from about 1140.degree. F. to about 1200.degree. F. and preferably from about 1150.degree. F. to about 1180.degree. F., and most preferably at about 1175.degree. F. Oxygen is supplied to the first stage in the form of fluidizing air. An excess of about 10% to about 500% air is utilized, about 30% or more air being preferred. In the second stage, the ore is heated to a temperature which allows retention of a maximum surface area while minimizing the carbon and sulfide content of the calcined ore. The second stage temperature, when phosphate rock is being processed, is preferably from about 1180.degree. F. to 1500.degree. F. and more preferably from about 1270.degree. F. to about 1330.degree. F. The heating of the second stage in the preferred embodiment is conducted in an atmosphere of excess oxygen, as supplied by air, of greater than about 50% excess air. Although the temperature ranges specified for each stage overlap, it will be apparent to those skilled in the art upon reading the disclosure that each stage is to be operated at a different temperature.
The phosphate rock produced has a low content of reduced sulfur compounds and thus is minimally corrosive when acidified. In addition, the carbon content has been reduced such that the adverse influence of residual carbon when the phosphate rock is acidulated has been reduced to an acceptable level, or a lower level. The product of the present process is also characterized by a high reactivity as measured by the rate of calcium and phosphate solubility.